The use of carbon fiber reinforced plastic (“CFRP”) materials, otherwise known as carbon fiber composites, for structural members is increasing in commercial airplanes because of the higher strength-to-weight and stiffness-to-weight ratios afforded by carbon fiber composites as compared to traditional aluminum structures.
A lightning strike to an aircraft may cause a high electric current, which may typically be of the order of 100,000 amps, to flow through the aircraft frame. In a carbon fiber composite structure, the carbon fiber plies may act as very high resistance conductors and the resin between the plies may act as highly capacitive dielectric layers so that lightning striking the carbon fiber composite results in an increasing potential difference produced across the ply structure but no readily available electrically conductive path for discharging the current. The current may concentrate at the fasteners joining between the skin panels and the aircraft substructure, since the fasteners are generally made of highly conductive alloys for strength. When the lightning energy is unable to dissipate at a fast enough rate, undesirable arcing and dangerous sparks may occur.
One known approach to reduce the effect of lightning strikes on non-metallic structures is to apply, by flame spraying or plating, a conductive layer such as aluminum to the outer surface of the skin panels. The problem with flame spraying or plating is that they are secondary operations used to apply the conductive layer to the already assembled structure. In addition, it is difficult to achieve a satisfactory electrical bond using either of these processes. The protective conductive layer also adds mass without contributing to the strength of the assembled structure, is vulnerable to environmental damage, and is difficult to maintain. The production and maintenance of such an arrangement is thus clearly expensive.
Another known approach to reduce the effect of lightning strikes on non-metallic surfaces is to install a conductive woven screen or foil to the outer surface of the skin panels. However, the use of woven screens or foils requires the use of an additional ply of fiberglass when the non-metallic structure is carbon to electrically isolate the carbon layer from the metal to prevent corrosion that also adds non-structural weight.
Yet another approach is to employ various special designs of fastener, for example as disclosed in U.S. Pat. No. 4,891,732. The fasteners disclosed in these patents employ a beveled head for countersinking into the surface of the skin panel into intimate contact therewith, and a nut for securing the fastener in place, which nut is designed to transmit electric current safely to the substructure such that arcing will not occur. Again, this approach to the problem of arcing is expensive because of the need for special fastener designs.
Still another known method for protecting fuel systems from lightning strikes is described in U.S. Pat. No. 5,845,872 to Pridham et al., which is herein incorporated by reference, which discloses a method for fastening an outer composite aircraft skin to an inner substructure including the steps of incorporating an electrically conductive layer in or applying it to an outer surface of the outer composite skin, inserting a bolt through the electrically conductive layer and outer composite skin and through the inner substructure, securing the bolt by means of a nut directly or indirectly engaging the inner surface of the inner substructure, and applying insulating material over the head of the bolt. While the Pridham disclosure effectively protects the fuel system from lightning strikes, the proposed system does not address the need for repairing the copper grid system after a lightning strike or after other mechanical damage. The electrical continuity of the electrically conductive layer must be reestablished to provide proper lightning strike protection.
There thus exists a need for an inexpensive and robust technique for repairing outer composite aircraft skins that utilize copper foil along fastener rows for lightning strike protection that have been damaged by lightning strikes or in some type of mechanical manner in order to maintain the integrity of the lightning prevention system to help divert lightning currents away from the fuel tank substructures.